It is well known that hydrophobicity may improve the mechanical properties of a surface. One of the crucial surface properties for materials in micro/nanoscale applications is non-wetting or hydrophobicity. Creating hydrophobic surfaces on materials is desirable in some applications, because these surfaces cause water to flow away from the surface, thereby preventing the buildup of liquid on the surface. Hydrophobic surfaces are also desirable due to their self-cleaning properties. These surfaces repel liquids, thereby resulting in liquid and contaminating particles flowing away from the surface.
Wetting is characterized by a contact angle, which is defined as the angle between the solid and liquid surfaces. If a liquid wets the surface, the value of the contact angle is 90° or less (referred to as wetting liquid), whereas if the liquid does not wet the surface (referred to as non-wetting liquid or hydrophobic surface), the value of the contact angle ranges between 90° and 180°. A surface is considered superhydrophobic, if the contact angle has a range of between about 150° to 180°.
Biomimetics has played a role in the development of new surfaces. Biomimetics, which comes from a Greek word “biomimesis” meaning to mimic life, describes the study and simulation of biological objects with desired properties. To that end, scientists have studied natural surfaces that are extremely hydrophobic, in order to reproduce these properties on artificial surfaces. Among these surfaces studied are the leaves of water-repellent plants such as Nelumbo nucifera (lotus). At least two surface characteristics are believed to produce water repellent properties on these surfaces. First, the surface of the leaves is usually covered with a range of different waxes made from a mixture of large hydrocarbon molecules, measuring about 1 nm in diameter, that are strongly hydrophobic. Second, the surface is very rough due to so-called papillose epidermal cells, which form asperities or papillae. The surface of the lotus leaf generally has pyramid shaped asperities that are spaced a few μm from one pyramid tip to another pyramid tip. Drops of water substantially contact only the tips or peaks of the pyramids so that the contact area of water to surface is minuscule relative to water drops contacting a micro smooth surface. The reduced contact surface area results in a very low adhesion between the water drops and the micro-rough surface.
Various methodologies have been developed for design and formation of super-hydrophobic surfaces which mimic that of the lotus leaf. For example, U.S. Patent Publication No. 2006/0078724 discloses design criteria for lotus leaf mimetic structures, and suggests a number of methods which could be used to make such structures, including etching and embossing processes, coating processes, shaping processes using appropriately structured molds, polishing processes, photolithography, solvent or vapor deposition, electroplating, electrowetting, plasma processing, warm-water processing, and high temperature sintering. However, no detailed method of formation is disclosed.
In “Fabrication of Hierarchical Structures on a Polymer Surface to Mimic Natural Superhydrophobic Surfaces”, Advanced Materials, vol. 19, pp. 2330-2335 (2007), Yuwon Lee et al. disclose a fabrication process consisting of three processes: photolithography, aluminum etching/anodization and polymer replication, wherein well-defined microstructure patterns were transferred onto the surface of an aluminum sheet by photolithography using a photoresist and shadow masks, followed by etching and anodizing of the aluminum surface to form a negative, hierarchical replication template, to which was applied a high density polyethylene substrate. The HDPE substrate was forced into the negative template under heat and pressure, and subsequently peeled from the template to produce lotus leaf mimetic hierarchical polymeric structures.
U.S. Patent Publication No. 2008/0217180 discloses a surface comprising a microstructure that reduces adhesion and to a method for producing said microstructure. Microstructures of this type that reduce adhesion are known and are used, for example, to configure self-cleaning surfaces that use the Lotus effect. According to the invention, the surface is produced electrochemically by means of reverse pulse plating, the known microstructure being first produced and a nanostructure that is overlaid on the microstructure is produced at the same time or in a subsequent step.
U.S. Patent Publication No. 2010/0098909 discloses an article having a nanotextured surface with superhydrophobic properties, comprising an array of vertical tabs, formed by photolithography.
In “Effect of Replicated Polymeric Substrate with Lotus Surface Structure on Adipose-Derived Stem Cell Behaviors”, Macromolecular Bioscience, vol. 11, pp. 1357-1363, Kyoung Je Cha et al. disclose fabrication of polystyrene substrates with lotus leaf surface structures by electroforming nickel onto a natural lotus leaf to form a mold, followed by hot embossing with polystyrene.
U.S. Patent Publication No. 2011/0177288 discloses methods of making superhydrophobic structures comprising depositing a polymer mold onto a silicon surface comprising a plurality of microasperities, removing the polymer mold after the polymer mold has hardened, depositing a liquid epoxy resin into the polymer mold, forming a microstructure with a plurality of microasperities by separating the epoxy resin from the mold after the epoxy resin has solidified, and forming a superhydrophobic structure by depositing a plurality of alkane nanoasperities on the microstructure in the presence of solvent vapor.
It would be desirable to provide a simpler method for formation of hierarchical, biomimetic structures.